Most thermally stable polymers, as a result of rigid aromatic units within the chain, are either partially crystalline, with the attending high crystalline transition temperatures, or else are amorphous but have high glass transition temperatures (Tg's). Both the crystalline and to some extent the amorphous materials tend to have poor solubility in common organic solvents. Those polyaromatics that are amorphous, and have relatively low Tg's, are processed readily but their use temperature is limited to the glass transition temperature, a temperature far below the thermal decomposition temperature.
In an effort to produce thermally stable polymers that could be fabricated readily and then converted to a material with high use temperatures, the applicant has in the past explored crosslinking reactions of aromatic polymers. Unfortunately, there are relatively few good crosslinking reactions of polyaromatics, in part because of their chemically inert structure and the reduced chain mobility. However, because biphenylene is known to react with aromatic structures to form stable links, applicant has been able in the past to effect the crosslinking reaction of polyaromatics by incorporating biphenylene directly into the polyaromatic main chain. Garapon, J.; Stille, J. K. Macromolecules 1977, 10, 627; Recca, A.; Garapon, J.; Stille, J. K. Macromolecules 1977, 10, 1344; Recca, A.; Stille, J. K. Macromolecules 1978, 11, 479; Stille, J. K. Pure Appl. Chem. 1978, 50, 273; Stille, J. K. Vysokomol. Soedin. 1979, 11, 2545. Thus, polyquinolines, polyamides, polyquinoxalines and polybenzimidazole, all containing biphenylene units within the main chain of the polymer, have been found to cross-link either thermally or with the aid of transition metal catalysts.
The resulting crosslinked polymers as mentioned above were insoluble in all solvents, had a higher storage moduli at ambient temperature, a higher storage moduli above the Tg, and a higher as well as a less intense (or nearly absent) Tg's as compared to the uncured polymer. From these reactions several observations were made: (1) In every case, crosslinking only took place above the Tg, even though the thermal or the transition metal catalyzed biphenylene reaction in some cases was known to take place at temperatures below the Tg; (2) Generally, the onset of the biphenylene exotherm, was above, but paralleled the Tg of the polymer. It had also been observed that in the presence of a transition metal catalyst such as a Ni(O) or Rh(I) complex, lower biphenylene reaction temperatures and shorter reaction times could be used to effect the same crosslinking reaction; (3) Relatively low incorporations of biphenylene (on the order of about 2.5 mole %) were sufficient to significantly raise the modulus of the cured polymer above its Tg; and (4) The thermal stability of the crosslinked polymers as measured by TGA was as good as the base or parent polymer which did not contain biphenylene units, indicating that crosslinks which were at least as stable as the types of structures in the parent polymer are formed and that reactions that generate unstable structures are not formed during crosslinking.
Although the crosslinking of a relatively wide variety of thermally stable polymers has been successfully accomplished by the incorporation of biphenylene units directly into the main polymer chain, there are a number of problems that have been encountered with this cross linking procedure, and for certain applications improvements in the curing reaction are desirable. Generally, shorter reaction times, and in some cases lower curing temperatures would be desirable.
One further specific problem in the past methods of crosslinking polyaromatics is the fact that the synthesis of biphenylene monomers for incorporation into a polymer main-chain requires the synthesis of difunctional biphenylene in "monomer grade" purity. Since these synthetic reactions generally do not process in high yields, the syntheses of simpler monofunctional biphenylene derivatives for use in crosslinking reactions in which high purity is not critical would be desirable.